BUILD: listener: fix build warning on global_listener_rwlock without threads

BUILD: listener: fix build warning on global_listener_rwlock without threads

The global_listener_rwlock was introduced by recent commit 13e86d947
("BUG/MEDIUM: listener: Fix race condition when updating the global mngmt
task"), but it's declared static and is not used when threads are disabled,
thus causing a warning to be emitted in this case. Let's just condition it
to thread usage to shut the warning.

This will need to be backported where the patch above is backported.